Measuring transformer

ABSTRACT

A measuring transformer comprises a measuring conductor carrying the current I m  to be measured, a pre-magnetizing winding which carries a pre-magnetizing current I v , and a magnetic field comparing means which is exposed to the magnetic field produced by the current I m  and the magnetic field produced by the pre-magnetizing current I v  and is alternately controlled in both directions of saturation. The magnetic field comparing means is a magnetic film which may be anisotropic, operated in the magnetic preferential direction, and have a thickness of at most a few microns. It may be secured to pole shoes of a magnetic core or arranged between a flat measuring conductor and a pre-magnetizing coil of flat cross-section. The measuring transformer can be used as an input transformer in a static electricity meter. In a further embodiment, the output pulses can be obtained directly from the magnetic field comparing means.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a measuring transformer for the potential-freemeasurement of currents or voltages, and to a static electricity meterincluding such a transformer.

2. Description of the Prior Art

FIG. 1 shows a known measuring transformer which comprises an annularmagnetic core 1, a measuring conductor 2 for carrying the current I_(m)to be measured, a pre-magnetizing winding 3 and an induction winding 4.The conductor 2 is passed through the closed magnetic circuit of themagnetic core 1, but could also be wound in a plurality of windingsaround the core 1, in the same manner as the pre-magnetizing winding 3.

In operation of this transformer, a pre-magnetizing current I_(v), whichis preferably of a triangular waveform and which flows through thepre-magnetizing winding 3, produces a magnetic field which alternatelydrives the magnetic core 1 in both saturation directions, the magneticcore 1 operating as a magnetic field comparing means. If the currentI_(m) to be measured is of zero value, a symmetrical induction voltageU_(a) is induced in the induction winding 4, and this induced voltagesubstantially comprises positive and negative pulses which occur at themoment of magnetism reversal of the magnetic core 1 and which follow atequal spacings in time. If, on the other hand, the instantaneous valueof the current I_(m) is greater than zero, then this current assists themagnetizing effect of the pre-magnetizing current I_(v), whereby thereis a temporal displacement of the positive and negative pulses of theinduction voltage U_(a). This temporal displacement can be evaluated asa measurement of the strength and direction of the current I_(m) to bemeasured. The induction winding 4 is not absolutely necessary, as avoltage is also induced in the premagnetizing winding 3, and thevariation of that voltage in time can be used in the same way as ameasurement with respect to the current I_(m).

A known measuring transformer supplies an induction voltage U_(a) whosepulses are relatively wide and have shallow flank angles of inclination.Moreover, the addition of the magnetic fluxes or magnetic fields whichare involved, in the region of saturation of the magnetic core 1, isdifficult to control, and this results in a complicated windingstructure or costly compensation operations. In addition, the temporaldisplacement of the pulses relative to the passage through zero of themagnetic field is relatively great. This situation is altered only to aminor extent if the magnetic core 1 has a reduced portion, to reduce thesaturation field strength. This known measuring transformer is thereforenot suitable for the precision measurement of currents which varyrapidly.

SUMMARY OF THE INVENTION

One object of the invention is to provide a measuring transformer of thekind set out above, whose magnetic field comparing means is virtuallydelay-free.

Another object of the invention is to provide a measuring transformer ofthe kind set out above, whose output pulses mark the moment of thepassage through zero of the magnetic field, clearly and with a highdegree of accuracy.

According to the present invention there is provided a measuringtransformer for the potential-free measurement of one of current andvoltage, the transformer comprising a measuring conductor for carrying acurrent to be measured, a pre-magnetizing winding for carrying apre-magnetizing current, and a magnetic field comparing means which isexposed to the magnetic field produced by the current to be measured andto the magnetic field produced by the pre-magnetizing current and isalternately controlled in both directions of saturation by the themagnetic field produced by the pre-magnetizing current, said magneticfield comparing means being a magnetic film of very small thickness incomparison with its length and width.

According to the present invention there is also provided a staticelectricity meter including an input measuring transformer for thepotential-free measurement of current, said transformer comprising ameasuring conductor for carrying the current to be measured by themeter, a pre-magnetizing winding for carrying a pre-magnetizing current,and a magnetic field comparing means which is exposed to the magneticfield produced by the current to be measured and to the magnetic fieldproduced by the pre-magnetizing current and is alternately controlled inboth directions of saturation by the magnetic field produced by thepre-magnetizing current, said magnetic field comparing means being amagnetic film of very small thickness in comparison with its length andwidth.

According to a further embodiment of the present invention, the outputpulses of the measuring transformer can be directly obtained from themagnetic field comparing means.

The above, and other objects, features and advantages of this inventionwill be apparent from the following detailed description of illustrativeembodiments which is to be read in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a known measuring transformer, referred to above;

FIG. 2 shows a magnetic film applied to a substrate;

FIGS. 3 to 7 show respective different embodiments of measuringtransformers with magnetic cores;

FIG. 8 shows a pulse diagram;

FIGS. 9 and 10 show current dividers for the current to be measured;

FIGS. 11 to 13 show respective different embodiments of measuringtransformers without magnetic cores;

FIG. 14 shows a further embodiment of a measuring transformer;

FIG. 15 shows parts of the measuring transformer of FIG. 14 in planview;

FIGS. 16 and 17 show variants of FIG. 15;

FIGS. 18 and 19 show further embodiments of a measuring transformer; and

FIG. 20 shows part of the measuring transformer of FIG. 19 in plan view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 2, reference numeral 5 denotes a preferably anisotropic magneticfilm whose thickness d is very small in comparison with the length h andthe width b. This magnetic film 5 which serves as a magnetic fieldcomparing means, is preferably applied to a non-magnetic substrate 6which imparts thereto the necessary mechanical strength and whichcomprises for example a glass or plastic plate. The film 5 may beapplied to the substrate 6 in accordance with known methods by vapordeposition in vacuum or by electrolytic coating. The film 5 can also be,for example, a foil which is produced by rolling and which is secured tothe substrate 6 by adhesive. Suitable materials for the film 5 are, forexample, known NiFe or NiFeCr magnetic alloys.

The anisotropic magnetic film 5 can be operated in the measuringtransformers described in greater detail hereinafter, in principle inthe magnetic preferential direction or in the non-preferentialdirection. When operating in the preferential direction, the coercivefield strength of the magnetic film 5 should be as low as possible andthe wall speed should be high. As low as isotropic field strength aspossible is advantageous when the magnetic film 5 is operated in thenon-preferential direction. The description of the function of theembodiments described hereinafter relates to operation in the magneticpreferential direction which has been found particularly advantageous.

In FIG. 3, the same components as in FIGS. 1 and 2 are denoted by thesame reference numerals. A magnetic core 7 which comprises ferromagneticmaterial of high permeability again carries the pre-magnetizing winding3 and engages in a tong-like configuration around the measuringconductor 2, but differs from the magnetic core 1 of FIG. 1 by an airgap 8 which is bridged by the magnetic film 5. The two longitudinal endsof the film 5 are secured, for example by adhesive, to the magnetic core7, on the outer surface thereof. The magnetic film 5 is advantageouslysubstantially longer than the air gap 8 so as to give the largestpossible contact surfaces 9 between the magnetic core 7 and the magneticfilm 5. If the magnetic film 5 is arranged on a substrate 6 (FIG. 2),the substrate (which is not shown in FIG. 3 for the sake of improvedclarity of the drawing) is advantageously disposed on the outsidesurface of the magnetic film 5, which is remote from the magnetic core7, so that there is no air gap at the contact surfaces 9 between themagnetic core 7 and the film 5. The width of the film 5 approximatelycorresponds to that of the magnetic core 7.

The above-described transformer operates as follows:

In the rest-condition the magnetic film 5 is saturated and itspermeability corresponds to that of a vacuum. By virtue of the currentI_(m) to be measured and the pre-magnetizing current I_(v) which flowthrough the magnetic core 7, a magnetic outer field H_(a) is built up inthe air gap 8 where the magnetic film is disposed, and the followingequation applies to the field H_(a), assuming ideal conditions: ##EQU1##where n_(v) represents the number of windings of the pre-magnetizingwinding 3, n_(m) represents the number of windings of the measuringconductor 2, and l represents the length of the air gap 8. As soon asthe outer field H_(a) exceeds the wall movement field strength of themagnetic film 5, a magnetism reversal process begins in the magneticfilm 5, and this process begins in the magnetic film 5, and this processmay be explained by the displacement of a Bloch wall. This displacementoccurs very rapidly so that the magnetization zero in the magnetic film5 in relation to the condition H_(a) =0 occurs, with only a very smalldelay. In this time interval the permeability of the magnetic film 5 isvery high, the magnetic circuit is closed to the maximum extent by wayof the magnetic film and the magnetic flux in the magnetic circuit risessteeply. This change in the magnetic flux is revealed in a steep jump involtage in the pre-magnetizing winding 3 and possibly in an inductionwinding 4 (FIG. 1). Thereafter the magnetic film 5 is saturated in theother direction, its permeability again corresponds to that of a vacuum,and the Bloch wall has passed through the entire width of the magneticfilm 5.

The following relationship applies for the effective magnetic fieldH_(eff) which switches the magnetic film 5: ##EQU2## wherein L is thelength of the magnetic circuit in the magnetic core 7, A is itscross-sectional area, a is the cross-sectional area of the magnetic film5, M_(s) is the saturation magnetization of the magnetic film 5, μ_(o)is the absolute permeability, and μ_(r) is the relative permeability ofthe magnetic core 7. The proportionality relationship:

    H.sub.eff ˜(n.sub.v ·I.sub.v +n.sub.M ·I.sub.m)=ΣI

is fulfilled if the second term in the above equation for H_(eff)disappears, that is to say, the relationship becomes:

    A>>a

and/or

    1· μ.sub.r >>L.

Observing the second inequality also provides the greatest possibleproportionality factor k=1/l between H_(eff) and ΣI, because thefollowing relationship then applies:

    1>>L/(1· μ.sub.r)

The current I_(m) to be measured and the pre-magnetizing current I_(v)are therefore converted into precisely proportional magnetic fields atthe location of the magnetic film 5 which operates as a magnetic fieldcomparing means, this being effected by way of the magnetic flux in themagnetic core 7. The moment of the passage through zero of this magneticfield is marked clearly and with a high degree of accuracy by an outputpulse which is very steep and which has a minimal delay relative to theoccurrence ΣI=0. In addition, the position in time of the output pulseis substantially independent of the angle at which the currents I_(m)and I_(v) cross in the current-time diagram.

The above-mentioned advantages are based on the particular magneticproperties which can be achieved with a very thin magnetic film, namelya small dynamic coercive field strength, a high switching speed of themagnetic film, low eddy current losses, low saturation field strength,low demagnetization (shift), low degree of dispersion of the magneticproperties within the small and thin magnetic film by virtue of highmetallurgical purity and homogeneity, and high uniaxial anisotropy.

The thickness d of the magnetic film 5 should be at most a few microns,in order to keep the saturation field strength, demagnetization, andeddy current losses as low as possible. A greater film thickness does infact result in a greater energy content of the output pulses, but hasits effect in particular in a widening in time and not in an increase involtage of the output pulses. The film thickness d is, in a particularlyadvantageous aspect, at most 2 microns; this gives negligible eddycurrent losses in the magnetic film 5 and thus a switching speed whichis limited only by material parameters of the magnetic film, such aswall mobility, purity, and the like.

Other advantages of the above-described measuring transformer lie in theease of production and the resistance of the mechanical parameters inregard to mechanical loadings, in the freedom from magnetostriction, thepossibility of continuous manufacture, and the way in which the magneticfilm 5 can be secured without problem, by adhesive and the like.

FIGS. 4 to 7 show advantageous forms of the pre-magnetizing winding 3,the induction winding 4 and the magnetic core, which can besubstantially combined together. In FIGS. 4 to 6, the pre-magnetizingwinding 3 and the induction winding 4 are in the form of cylindricalcoils which are arranged axially one beside the other in FIGS. 4 and 5and concentrically in FIG. 6. In FIG. 7, the induction winding 4 passesaround the magnetic film 5 in the region of the air gap, wherebycross-talking of the pre-magnetizing current I_(v) to the inductionwinding 4 is substantially prevented.

The magnetic core 10 shown in FIG. 4 comprises a U-shaped member withinwardly bent pole shoes 11 and 12, the magnetic film 5 being secured tothe pole surfaces of the pole shows 11 and 12, which surfaces lie in acommon plane. The magnetic core 13 shown in FIG. 5 also comprises aU-shaped member with inwardly bent pole shoes 14 and 15, but in thiscase the inner surfaces 16 and 17 of the ends of the pole shoes 14 and15 again extent parallel to the limbs of the U-shaped member. This makesit possible to avoid saturation phenomena in the pole shoes 14 and 15.

The U-shaped magnetic core 18 shown in FIGS. 6 and 7 does not have anypole shoes; the length of the air gap approximately corresponds to thecoil width of the pre-magnetizing winding 3.

In the above-described measuring transformers, the number of windingsn_(v) of the pre-magnetizing winding 3, the number of windings n_(s) ofthe induction winding 4 and the length l of the air gap 8 (FIG. 3) maybe selected substantially independently of each other. Thepre-magnetizing current I_(v) whose amplitude is advantageously notgreater than some ten milliamps, in order to avoid expensive apparatusfor the production thereof, is adapted to the measuring current I_(m),with the number of windings n_(v). The number of windings n_(s)determines the magnitude of the induced output voltage U_(a). The fieldstrength H_(a) produced in the air gap 8 is determined by the selectionof the air gap length l.

FIG. 8 shows the form of the output voltage U_(a) against time t, whichwas determined in a measuring transformer as shown in FIG. 4, with thefollowing data:

Material of the magnetic core 10: Ferrite

Material of the magnetic film 5: NiFe

Length h of the magnetic film 5: 5 mm

Width b of the magnetic film 5: 1 mm

Thickness d of the magnetic film 5: 1.5 micron

Length l of the airgap: 1 mm

Number of windings n_(v) of winding 3: 250

Number of windings n_(s) of winding 4: 250

Amplitude of the current I_(v) : 20 mA

Frequency of the current I_(v) : 1 kHz

Measurements were effected with an amplitude of the output pulse of 30mV, a rise time t_(r) of 5 μs, a decay time t_(f) of 11 μs and a pulseduration t_(p) of 10 μs.

In order to be able to measure very high currents with the measuringtransformer, while nonetheless keeping the number of windings n_(v) andthe pre-magnetizing current I_(v) within acceptable limits, it may beadvantageous for the current to be measured to be divided by means of acurrent divider into the current I_(m) and a shunt current. FIGS. 9 and10 show advantageous examples of such a current divider.

The current divider 19 shown in FIG. 9 comprises a single metal platewhich has current connections 20 and 21, and a cut 22 which extendslongitudinally relative to the direction of current flow and whichdivides the middle region of the metal plate into a measuring currentpath 23 and a shunt current path 24. The two current paths 23 and 24 arebulged outwardly in opposite directions to a semicircular form and forman eye into which the magnetic core 7, 10, 13 or 18 can be inserted insuch a way that the magnetic circuit of the magnetic core encloses themeasuring current path 23.

FIG. 10 shows a current divider 25 which also comprises a single metalplate which however in this case is flat, provided with currentconnections 26 and 27, a shunt current path 28 and a measuring currentpath 30 which is separated from the current path 28 by a punched-outportion 29. In this embodiment the magnetic core 7, 10, 13 or 18 isinserted into the portion 29 so that the magnetic circuit of themagnetic core encloses the measuring current path 30.

Making the current divider 19 or 25 in the form of an integral metalplate ensures a constant current divider ratio which is independent ofambient influences. The phase displacement caused by the current divider19 or 25 respectively: ##EQU3## (ω=circuit frequency, L=inductance ofthe transformer, R=resistance of the measuring current path 23 or 30)can be kept small if the metal plate is of small cross-section, and thusthe measuring current path is of high resistance R, and the inductance Lis as small as possible, by suitable dimensioning of the measuringtransformer. A certain degree of compensation for the phase displacementφ is already achieved by the finite switching speed of the magnetic film5; any additional compensation which may be necessary can be achievedwith simple phase displacer members or by covering the shunt path 24 or28 by a soft-magnetic layer of suitable thickness.

As already mentioned, in the above-described measuring transformers,conversion of the current I_(m) and the pre-magnetizing current I_(v)into proportional magnetic fields is effected by way of the magneticflux in a magnetic core. Some embodiments are described hereinafter, inwhich the current I_(m) and the pre-magnetizing current I_(v) areconverted directly to proportional magnetic currents at the position ofthe magnetic film 5, so that no magnetic core is required.

In FIG. 11, a measuring conductor 31 for carrying the current I_(m) tobe measured is in the form of a flat conductor. The pre-magnetizingwinding 3 is formed by a disc-shaped flat coil 32. The magnetic film 5is arranged between the conductor 31 and a part, parallel thereto, ofthe flat coil 32, in a region in which both the magnetic surface fieldof the conductor 31, which is produced by the current I_(m), and alsothe magnetic surface field of the flat coil 32, which is produced by thepre-magnetizing current I_(v), are uniform. Such a region of uniformmagnetic field, which region is solely dependent on geometric factors,can be provided if the conductor 31 and the part of the flat coil 32which is parallel thereto are as closely adjacent as possible and havethe flattest possible cross-section, that is to say, a thickness d₁ andd₂ which is small in comparison with the width b₁ or b₂, respectively.

The flat coil 32 can be made in the form of a self-supporting coil orstrip or of wire, with one or more windings for each coil winding layer.In addition, the flat coil 32 may comprise one or more conductor plateswhich have a spiral copper layer on one or both sides, in the manner ofan etched printed circuit. The magnetic film 5 can be secured forexample directly to the measuring conductor 31 by adhesive.

The transformer shown in FIG. 12 differs from that shown in FIG. 11 onlyinsofar as the pre-magnetizing winding 3 is formed by a flat cylindricalcoil 33, while the magnetic film 5 is disposed between the conductor 31and one flat side of the coil 33. At the position of the magnetic film5, the magnetic outer field of the coil 33 and the magnetic surfacefield of the conductor 31 are superimposed upon each other.

In FIG. 13, a flat cylindrical coil 34 forms the pre-magnetizing windingand passes around the magnetic film 5. A flat conductor 35 which carriesthe current I_(m) passes in a loop around the flat cylindrical coil 34.The magnetic inner field of the coil 34 and that of the loop formed bythe conductor 35 are superimposed on each other at the location of themagnetic film 5. A magnetic short circuit 36 comprising a material witha high degree of permeability provides for a magnetic connection betweenthe two opposite ends of the magnetic film 5 which projects out of thecoil 34, and thereby reduces demagnetization of the magnetic film 5.

The above-described measuring transformers are used for thepotential-free measurement of direct or alternating currents. By seriesconnection of a high-value resistance with the measuring conductor, byreplacing the measuring conductor by a winding with a suitably highnumber of windings, or by combining the two possibilities justmentioned, they can also be used for measuring direct or alternatingcurrent voltages. They provide very steep and narrow output pulses whosedisplacement in time can be used as a measurement of the instantaneousvalue of the magnitude and direction of the electrical signal to bemeasured. The above-described measuring transformers are advantageouslyused as input transformers in static electricity meters.

In the measuring transformers described above, with the moment ofpassage through zero of the magnetic field, produced by thepre-magnetizing current and the current to be measured, an outputimpulse is induced in the pre-magnetizing winding or in a separateinduction winding, thus marking the passage through zero of the magneticfield with great accuracy. However, said transformer has a disadvantagein that the premagnetizing winding or the induction winding, from whichthe output impulse is obtained, is inductively coupled with themeasuring conductor. High frequency interference signals, flowing in themeasuring conductor, are therefore inductively transferred to thepre-magnetizing winding, serving as the output winding, where they aresuperimposed on the output impulses. In an evaluating circuit, connectedto the measuring transformer, such interference signals cannot bereadily differentiated from the output signals marking the passagethrough zero of the magnetic field. A suppression of the interferencesignals in the evaluating circuit is not possible if the spectrum ofsaid signals is the same as that of the output impulses, or is in theproximity thereof.

In a further embodiment of the instant invention the output pulses canbe directly obtained from the magnetic field comparing means. As greatas insensitivity as possible to interference signals is achieved withthe measuring transformer in accordance with this further embodiment. Achange in resistance of the magneto-resistive magnetic film occurs onlyat the moment of passage through zero of the magnetic field. To have anyeffect at all, the interference signals must either occur in theproximity of this moment, or must be very strong in order to bring abouta change in the magnetizing direction of the magnetic film, which isstatistically less probable than the above-mentioned case ofinterference. A further advantage is that the pre-magnetizing current inthe magneto-resistive magnetic film does not give rise to aninterference signal which superimposes itself on the output impulsesobtained at the contacts of said magnetic film. This also affords theadvantage that the premagnetization current does not have to obey acontinuous function, but can be a step-shaped signal, for example.

Referring to FIG. 14, reference numeral 41 indicates a magnetic core offerromagnetic material whose magnetic circuit encloses the air gap 43between two pole shoes 42, whereby the pole surfaces 44 of the poleshoes 42 lie in a common plane. The magnetic core 41 engages in atong-like configuration around the measuring conductor 45 which carriesthe current I_(m) to be measured. Moreover, the magnetic core 41 carriesa pre-magnetizing winding 46 through which, for example, atriangular-shaped pre-magnetizing current I_(v) flows.

The following described parts of the measuring transformer are drawn inan exploded view in FIG. 14 to give better lucidity. A magnetic film 48of ferromagnetic, magneto-resistive material, whose thickness is verysmall in comparison with its length and width, is arranged on anon-magnetic, electrically insulating substrate 47. Said film isadvantageously magnetic, anisotropic or uniaxial. The magnetic film 48may be applied to the substrate 47 in accordance with known methods byvapor deposition in vacuum or by electrolytic coating.Photo-lithographic methods can be used, for example, for designpurposes. NiFe alloys and triples derived therefrom (e.g., NiFeCr orNiFeCo) or higher alloys are preferably suited as magneto-resistivematerials. The active length and width of the magnetic film 48corresponds to the dimensions of the air gap 43, e.g., each 1 mm. Thetypical thickness of the magnetic film 48 lies in the order of 40 nm. Inorder to avoid extremely low values for the thickness of the magneticfilm 48 and nevertheless to achieve suitably high resistance values fordetection of the change in resistance, the magnetic film can be formedin a meander-like design. The longitudinal ends of the magnetic film 48are coated, for example, with a 100 nm thick conducting layer 49 ofgold, copper or the like, the outer ends of which each carry a contact50 made of a good conducting material. The pole surfaces 44 of magneticcore 41 lie above the conducting layer 49 so that the magnetic film 48,which practically lies on a plane with the pole surfaces, bridges theair gap 43. The preferential magnetic direction (easy axis) of themagnetic film 48 can lie parallel to, vertical to or, for example, at anangle of 45° to, the direction of the magnetic field in the air gap 43.The direction of the current flowing in the magnetic film 48, andproduced by the current or voltage source connected to the contacts 50,is parallel to the direction of the magnetic field in the depictedexample.

In FIG. 15 can be seen the arrangement of the magnetic film 48, the polesurfaces 44, the conducting layer 49 and the contacts 50 viewed from theside of the magnetic core 41. The above-described measuring transformeroperates as follows: In the rest condition, the magnetic film 48 has aconstant ohmic resistance in the order of, for example, 100Ω. A magneticfield is built up in the air gap 43 of magnetic core 41 by the currentto be measured I_(m) and the pre-magnetizing current I_(v). The magneticfilm 48 changes its resistance discontinuously at each passage throughzero of said magnetic field. If a current or voltage source is connectedto the contacts 50, then said change in resistance manifests itself inthe form of a needle-shaped voltage or current impulse which marks themoment of passage through zero of the magnetic field unequivocally andwith great accuracy. Since the magnetic film 48 is controlled up tosaturation, the height of the output impulse is independent of thestrength of the magnetic field. If the instantaneous value of thecurrent to be measured I_(m) differs from zero, then its magnetizingeffect superimposes itself on that of the pre-magnetizing current I_(v),whereby the output impulses are displaced with respect to time. Thisdisplacement with respect to time can be interpreted, in an evaluationcircuit connected to contacts 50, as a measure of the strength anddirection of the current to be measured I_(m). The coupling in of themagnetic field into the magnetic film 48 is most effective when carriedout in the preferential magnetic direction of the magnetic film. Thechange in resistance achieved thereby is at its lowest. It can beincreased if, in accordance with FIG. 16, strips 51 of gold, or anothermaterial with good electrical conducting properties, are applieddiagonally at 45° to the active surface of the magnetic film 48. Such a"barber's pole" arrangement of the magnetic film 48 causes the currentdirection in the said magnetic field to be rotated by 45°.

If the preferential magnetic direction of the magnetic film 48 does notrun parallel to the direction of the magnetic field, but forms an angleof, for example, 90° or 45° to the magnetic field, then a crossedconfiguration in accordance with FIG. 17 is advantageous. In comparisonwith FIG. 15, in FIG. 17 the magnetic film 48, together with theconducting layers 49 and the contacts 50, are so rotated in the drawingplane that the current in the magnetic film 48 flows in a verticaldirection to the magnetic field.

The configuration according to FIG. 18 differs from that according toFIG. 14 by virtue of a magnetic layer 52 arranged between the magneticfilm 48 and the substrate 47, and insulated from said magnetic film 48by means of a very thin insulating layer 53. The magnetic layer 52likewise comprises a ferromagnetic NiFe alloy, but is essentiallythicker than the magnetic film 48. The typical thickness of the magneticlayer 52 is 1 to 2 microns. The magnetic layer 52 also enables a goodcoupling in of the magnetic field when said magnetic field does not runin the preferential axial direction of the magnetic field 48. On thebasis of the magnetic coupling between the magnetic layer 52 and themagnetic film 48, a larger change in resistance of said magnetic film 48results during the passage through zero of the magnetic field.

The magnetic layer 52 can be preferably applied with a crossedconfiguration according to FIGS. 19 and 20. With this arrangement, themagnetic film 48 is positioned in the same manner as with the crossedconfiguration according to FIG. 17, namely directly on the substrate 47,and said arrangement also boasts the conducting layers 49 and thecontacts 50, whereby the direction of the current in the magnetic film48 is vertical to the direction of the magnetic field between the polesurfaces 44. The preferential magnetic direction of the magnetic film 48is likewise vertical to the direction of the magnetic field. Themagnetic layer 52 rests on the pole surfaces 44, bridges the air gap 43and crosses the magnetic film 48 at right angles. The preferentialmagnetic direction of the magnetic layer 52 is parallel to the directionof the magnetic field. The magnetic film 48 lies below the magneticlayer 52, whereby magnetic film 48 and magnetic layer 52 areelectrically insulated from each other by means of a very thininsulating layer not shown in the drawing. As a result of the magneticcoupling between the magnetic layer 52 and the magnetic film 48, themagnetizing of the magnetic film 48 in the passage through zero of themagnetic field is rotated so that a strong change in resistance inascertainable at the contacts 50. Preferably the magnetic film 48 forms,together with one or three resistors, a voltage divider or a bridgecircuit respectively. These resistors are advantageouslymagneto-resistive magnetic films of the same type as magnetic film 48,so that temperature influences are compensated. Furthermore, theseresistors can likewise be exposed to the magnetic field of the magneticcore 41 so that their output signals superimpose themselves in a manneras to be advantageous for the evaluation. Of course, the above-describedmagneto-resistive magnetic film 48, provided with contacts, can also beapplied with measuring transformers in accordance with FIGS. 11 to 13above, which have no magnetic cores.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in the art without departing from the scope andspirit of the invention as defined by the appended claims.

What is claimed is:
 1. A measuring transformer comprising:a core of magnetizable material having an air gap; an anisotropic thin film of magnetizable material positioned at said gap, a surface of said film being parallel to a path of magnetic flux bridging said gap; a measuring conductor for carrying a current to be measured, said conductor being disposed adjacent said core for magnetic coupling therewith; and a pre-magnetizing winding for carrying an alternating pre-magnetizing current to induce an alternating varying magnetic field which repetitively saturates the material of said film in both directions of saturation, said winding being disposed adjacent said core for magnetic coupling therewith, whereby the point of time of the saturation of said film is dependent on the strength of said current to be measured.
 2. A measuring transfomer according to claim 1 wherein said film is deposited on a substrate.
 3. A measuring transformer according to claim 1 wherein said film is a rolled foil.
 4. A measuring transformer according to claim 1 wherein the thickness of said film is at most a few microns.
 5. A measuring transformer according to claim 1 wherein the product of the gap length and the relative permeability of said core is much greater than the length of the magnetic circuit of said core.
 6. A measuring transformer according to claim 1 wherein said core has two pole shoes and said film has two ends which are secured to respective ones of said pole shoes.
 7. A measuring transformer according to claim 1 wherein an induction winding passes around said film.
 8. An electric meter including an input measuring transformer for the measurement of current, said transformer comprising:a core of magnetizable material having an air gap; a measuring conductor disposed adjacent said core for carrying the current to be measured by the meter; a pre-magnetizing winding disposed adjacent said core for magnetic coupling therewith for carrying an alternating pre-magnetizing current; and an anisotropic thin film of magnetizing material positioned at the air gap of said core and responsive to a magnetic flux of said current to be measured and responsive to a magnetic flux of said pre-magnetizing current to become saturated in alternate directions of magnetization corresponding to alternating directions of said pre-magnetizing current whereby the point of time of the saturation of said film is dependent on the strength of the current to be measured.
 9. A measuring transformer for the measurement of current or voltage comprising:a magnetic core having an air gap; a measuring conductor for carrying a current to be measured; a pre-magnetizing winding for carrying an alternating pre-magnetizing current, said measuring conductor and said pre-magnetizing winding being positioned relative to said magnetic core for linking magnetic fluxes of their respective currents with said magnetic core; and an anisotropic magnetic thin film comprising a ferromagnetic, magnetoresistive material having contact means for the connection of a current or voltage source, said air gap being bridged by said magnetic film, a surface of said film being parallel to a path of magnetic flux bridging said air gap, said film being linked by each of said magnetic fluxes for providing states of magnetization in alternate directions corresponding to alternate senses of said pre-magnetizing current whereby the point of time of the saturation of said film is dependent on the strength of the current to be measured.
 10. A measuring transformer according to claim 9 wherein said magnetic film has a meander-line form.
 11. A measuring transformer according to claim 9 wherein said magnetic film is coated with diagonal strips of electrically conductive material.
 12. A measuring transformer according to claim 9 wherein said magnetic film is magnetically coupled with a magnetic layer which is thicker than said magnetic film. 